1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same and, in particular, to a semiconductor device having a heat-radiation surface for radiating heat to a cooler and a method of manufacturing the same.
2. Description of the Background Art
Since a power module easily generates a large amount of heat, the power module is mounted on a cooler such as a cooling fin or a heat sink. Since efficient heat radiation cannot be achieved due to the presence of a gap between the power module and the cooler, in general, heat-conductive grease is applied between a metal base plate which the power module has and the cooler. In terms of an applying operation, grease having relatively high fluidity is generally used. As a material of the heat conductive grease, a material having relatively high heat conductivity can be selected. However, since the heat conductivity is lower than those of the materials of the base plate and the cooling fin, the grease is desired to properly bury the gap between the base plate and the cooling fin without an excessive amount. For this purpose, a technique for controlling a thickness distribution of the grease has been examined.
For example, according to Japanese Patent Application Laid-Open No. 2000-58727, a technique for suppressing the thickness of grease to a low level at the center of the module has been examined. In this technique, projections are arranged on a peripheral side of mounting holes of a metal base. With this configuration, a moment is generated on the module by using a fastening force of a mounting screw, and the center of the metal base is brought close to the heat sink. As a result, the thickness of grease at the center is reduced.
According to the technique described in the publication, when a metal base (base plate) is mounted on a heat sink (cooler), the posture of the base plate with reference to the cooler is regulated by projections arranged on the peripheral side of the base plate. For this reason, the posture of a central portion possessing a large part of the base plate is only indirectly controlled through a moment of a module, and is not directly regulated. As a result, deviated stress easily occurs when the base plate is attached to the cooler. Due to the deviated stress described above, a thickness distribution of grease may be disturbed. Even though the thickness distribution of grease is proper immediately after the attaching operation, through a thermal cycle occurring with an operation of the power module, the thickness distribution of grease may be easily disturbed due to deviated stress. As described above, in the conventional technique, thermal and mechanical contacts between the base plate and the cooler may be easily insufficient.